The invention relates to a technique for detecting the rotational speed (e.g., RPMs) and the angular position of a rotating wheel with a non-contact sensing device.
For electronic control systems in motor vehicles (e.g., an ABS braking system or an electronic ignition system) it is necessary to measure parameters such as rotational speed, the relative angular position of a rotating wheel or the crankshaft and/or their angular acceleration or instantaneous angular velocity. This is often performed by a non-contact sensor (e.g., an optical sensor or a Hall sensor) that scans the rotating wheel or a wheel seated on the crankshaft. The rotating wheel typically includes scan marks (e.g., lines or teeth).
The accuracy of the rotational speed and angular position measurement is often impaired by external noise signals and/or thermal noise. In a measurement of angular position, the thermal noise is often noticeable as jitter. If a digital measurement system is used to measure the rotational speed and the angular position, there is another error source in addition to the thermal noise that garbles the measurement result. This error source is determined by the minimum digital resolution of the least significant bit (LSB) or by the clock pulse rate. Usually the measurement error caused by the minimum digital resolution of the LSB is greater than the measurement error caused by thermal noise. For example, if the position of the threshold changes within one revolution, the scanning times between the individual scan marks on the wheel also change, so that the valuation device sees an instantaneous change of the rotational velocity, although the rotational velocity has not changed.
Other error sources are the offset of the sensor and the long-term drift due to aging of the sensor, or a gradual change of temperature in the measurement environment (e.g., by the warming of the engine).
Therefore, there is a need for a technique for compensating for various error sources associated with sensing rotational speed and/or angular position.